Loading…
Loading…
Loading…
OCR GCSE Chemistry A · J248
OCR J248 · C3.1c Check the specification (PDF) (opens in a new tab)
A chemical equation uses symbols and formulae to show the substances that react and the new substances formed. To write one, start by finding each element’s name and symbol on the . A symbol has one or two letters: the first is a capital and the second, if present, is lower-case. For example, magnesium is Mg and chlorine is Cl. Some symbols are less like their English names, such as sodium (Na), potassium (K), iron (Fe) and copper (Cu), so checking the table matters.
The first 20 elements are a useful starting set. In atomic-number order they are hydrogen H, helium He, lithium Li, beryllium Be, boron B, carbon C, nitrogen N, oxygen O, fluorine F, neon Ne, sodium Na, magnesium Mg, aluminium Al, silicon Si, phosphorus P, sulfur S, chlorine Cl, argon Ar, potassium K and calcium Ca. You also need to use the names and symbols of relevant elements beyond these, including those in Groups 1, 7 and 0 and other common elements encountered in the specification. The table helps you identify an element; its position can also give a clue about how it occurs and reacts.
A symbol names an element, but a formula represents a substance. For metals such as sodium and magnesium, the element’s formula is simply its symbol: Na or Mg. The Group 0 noble gases, including helium, neon and argon, exist as individual atoms, so their formulae are He, Ne and Ar.
Several non-metal elements occur as molecules containing two atoms. Hydrogen, nitrogen, oxygen and the Group 7 halogens fluorine, chlorine, bromine and iodine are written H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. The small ₂ is part of each formula: O₂ means one molecule made of two oxygen atoms. Group 1 includes lithium, sodium and potassium; Group 7 includes the halogens; and Group 0 includes the noble gases. Recognising these groups helps you select the correct elemental formula when a reaction names an element.
For a compound, its formula shows which elements are combined. Water is H₂O: each molecule contains two hydrogen atoms and one oxygen atom. Magnesium oxide is MgO. Use the correct compound formula when writing an equation; working out unfamiliar ionic compound formulae from ion charges is developed separately.
Write the reactants’ formulae on the left of an arrow and the products’ formulae on the right. The arrow means ‘react to form’. For magnesium reacting with oxygen to make magnesium oxide, begin with Mg + O₂ → MgO. The formulae identify the substances, but this version has two oxygen atoms on the left and only one on the right.
Atoms are rearranged in a reaction, not created or destroyed. Put a coefficient in front of MgO to make two units of it, then put a coefficient in front of Mg to balance magnesium:
There are now two magnesium atoms and two oxygen atoms on each side. The coefficient 2 in front of MgO multiplies all atoms in MgO; it does not change what magnesium oxide is. This is why a balanced equation represents both the substances involved and the conservation of atoms—and therefore mass—during the reaction.
Use the supplied Periodic Table for element names and correctly capitalised symbols, including the first 20 elements and relevant members of Groups 1, 7 and 0. Metals and Group 0 gases have elemental formulae such as Mg and Ne. Hydrogen, nitrogen, oxygen and the Group 7 halogens have diatomic formulae: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
Write the correct reactant and product formulae first. Then adjust coefficients, not subscripts, until each element has the same number of atoms on both sides: . The equal atom counts express conservation of mass.
Use the supplied Periodic Table to check symbols, including capital letters: Co is cobalt, but CO is carbon monoxide.
Check whether an element is written as single atoms or diatomic molecules before balancing. Chlorine is Cl₂, not Cl.
Balance by changing numbers in front of formulae, never the small numbers within them. Count each type of atom on both sides at the end.
It can see this note and what your specification asks for.
Answers are free with an account.
1 free note left this week
Get more free notesYour answer sets when this note comes back for review, or marks it complete.
Was this note helpful?
Log in to tell us whether it helped, or to report a problem.
Last updated
Chemical symbol
A one- or two-letter abbreviation for an element, beginning with a capital letter.
Chemical formula
A representation of a substance that shows its elements and how many atoms of each are present, or their simplest ratio.
Diatomic molecule
A molecule containing two atoms, such as O₂ or Cl₂.
Balanced chemical equation
A chemical equation with the same number of atoms of each element on both sides.
Coefficient
A number placed before a chemical formula that multiplies the whole formula in an equation.
Put your knowledge into practice — try past paper questions for Chemistry A
Chemical symbol
A one- or two-letter abbreviation for an element, beginning with a capital letter.
Chemical formula
A representation of a substance that shows its elements and how many atoms of each are present, or their simplest ratio.
Diatomic molecule
A molecule containing two atoms, such as O₂ or Cl₂.
Balanced chemical equation
A chemical equation with the same number of atoms of each element on both sides.
Coefficient
A number placed before a chemical formula that multiplies the whole formula in an equation.